Yes, the statement, about USB external installs not working unless your Mac doesn't have an internal, is incorrect.
You can make a bootable flash drive from your Snow Leopard DVD.
One method that will do that:
You need a working DVD drive - another Mac, or an external drive will work fine.
Disk Utility, Restore.
Your Snow Leopard DVD is the Source, and a USB flash drive is the Destination. Click the Restore button, and wait until complete.
I use an 8GB flash drive. The result is an installer that is more likely to work next time you need it. (flash drives don't mind bouncing around in a drawer, and scratches are not an issue ) and, the install is almost always faster than from a DVD.

1. On the MacBook (as it is now), create a new user account. Give it a simple name, such as "Administrator", and the password of your choice.
2. Connect the first flashdrive
3. Use CarbonCopyCloner to create a "selective clone" to the flashdrive. By "selective clone", go into CCC's "left menu" and chose NOT to copy your actual user account (which probably has too much "stuff" to fit onto the flashdrive). The same for Applications -- you can go in and DE-select everything except the essential Apple apps.
The idea is to create a bootable but "slimmed down" copy of your internal drive so that it will fit onto the first USB drive.
4. When done, DO A TEST BOOT of the new flashdrive by rebooting with the option key held down until startup manager appears, then select the flashdrive and try to boot from it.

Does this work?
If so, next step:

1. You will need to get a copy of the Snow Leopard DVD onto the -other- flashdrive. If you have access to another Mac that has a DVD drive that will read the DVD, use that Mac to copy the contents from the DVD to the second flashdrive.
(NOTE: if you do NOT have access to another Mac, there are "alternative download methods" by which to obtain a copy of the 10.6 DVD dmg, but you will have to seek them out and discover them on your own...)
2. Now, boot the MacBook from the FIRST flashdrive to your new "Administrator" account.
3. Use Disk Utility to re-initialize the internal drive. Use the secure erase if you wish.
4. When that's done, connect the SECOND flash drive with 10.6 on it.
5. Try running the 10.6 installer, and "aim it" at the internal drive.
6. Can you get a fresh copy of 10.6 installed onto the internal this way?

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